Visible-light-induced water oxidation mediated by a mononuclear-cobalt(II)-substituted silicotungstate

A mononuclear-cobalt(II)-substituted silicotungstate, K10[Co(H2O)2(γ-SiW10O35)2]⋅23 H2O (POM-1), has been evaluated as a light-driven water-oxidation catalyst. With in situ photogenerated [Ru(bpy)3](3+) (bpy=2,2'-bipyridine) as the oxidant, quite high catalytic turnover number (TON; 313), turnover frequency (TOF; 3.2 s(-1)), and quantum yield (Φ(QY); 27%) for oxygen evolution at pH 9.0 were acquired. Comparison experiments with its structural analogues, namely [Ni(H2O)2(γ-SiW10O35)2](10-) (POM-2) and [Mn(H2O)2(γ-SiW10O35)2](10-) (POM-3), gave the conclusion that the cobalt center in POM-1 is the active site. The hydrolytic stability of the title polyoxometalate (POM) was confirmed by extensive experiments, including UV/Vis spectroscopy, linear sweep voltammetry (LSV), and cathodic adsorption stripping analysis (CASA). As the [Ru(bpy)3](2+)/visible light/sodium persulfate system was introduced, a POM-photosensitizer complex formed within minutes before visible-light irradiation. It was demonstrated that this complex functioned as the active species, which remained intact after the oxygen-evolution reaction. Multiple experimental parameters were investigated and the catalytic activity was also compared with the well-studied POM-based water-oxidation catalysts (i.e., [Co4(H2O)2(α-PW9O34)2](10-) (Co4-POM) and [Co(III)Co(II)(H2O)W11O39](7-) (Co2-POM)) under optimum conditions.